Trostani's first ability checks the creature's toughness as it resolves. If that creature has left the battlefield, use its toughness from when it was last on the battlefield. You can't lose life this way if that creature's toughness was less than 0.

4/15/2013

You can choose any creature token you control for populate. If a spell or ability puts a token onto the battlefield under your control and then instructs you to populate (as Coursers' Accord does), you may choose to copy the token you just created, or you may choose to copy another creature token you control.

4/15/2013

If you choose to copy a creature token that's a copy of another creature, the new creature token will copy the characteristics of whatever the original token is copying.

4/15/2013

The new creature token copies the characteristics of the original token as stated by the effect that put the original token onto the battlefield.

4/15/2013

The new token doesn't copy whether the original token is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any noncopy effects that have changed its power, toughness, color, and so on.

4/15/2013

Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the new token will work.

4/15/2013

If you control no creature tokens when you populate, nothing will happen.